The present invention relates to a method and an apparatus for controlling an air-fuel ratio in internal combustion engines.
In a conventional electronic fuel injection control system, an oxygen concentration is detected to generate a feedback signal so as to control the fuel injection amount and hence the air-fuel ratio. The air-fuel ratio must be very precisely controlled by the feedback signal so that it falls within a narrow range having a stoichiometric air-fuel ratio as its center.
However, the basic air-fuel ratio often deviates from the stoichiometric air-fuel ratio in accordance with various factors. In this case, the control precision of the stoichiometric air-fuel ratio is decreased at the transient state, and the exhaust gas components are degraded. In order to eliminate this drawback, a learning control system is proposed to automatically correct the basic air-fuel ratio.
In this learning control system, the deviation of the basic air-fuel ratio with respect to the stoichiometric air-fuel ratio is detected as correction data or amount for every given rotational frequency by a control voltage level during the feedback control operation. The correction data is added to the basic data so as to correct the deviation of the basic air-fuel ratio.
When an automobile ascends or descends a high mountain, the correction data for feedback control greatly changes with respect to deviation in the air-fuel ratio. The correction data is stored in a memory. The correction data is then further updated as needed. However, when the automobile is running only along a sea level road, the corresponding correction data is obtained and is updated in the nonvolatile memory. In this case, correction data corresponding to any other operating state is not updated. As a result, proper correction data cannot be obtained for such other operating states. In this manner, when the operating conditions greatly change, the basic air-fuel ratio cannot follow the stoichiometric air-fuel ratio with high precision. As a result, effective air-fuel ratio control cannot be performed.